1. Field of the Invention
The present invention relates to a surface acoustic wave element, a method of producing the surface acoustic wave element, and a method of changing a resonation frequency of the surface acoustic wave element.
2. Description of Related Art
A surface acoustic wave element has a comb-teeth electrode and a reflector, which are arranged on a piezoelectric member. The comb-teeth electrode and the reflector are formed to have a predetermined electrode pattern by using a photo etching. After an aluminum membrane is formed on the piezoelectric member by sputtering, the photo etching is performed. The photo etching is defined by an etching process using a photolithography. An electrode interval of the comb-teeth electrode and the reflector has a predetermined relationship with a resonation frequency. Therefore, the electrode interval is set to have a target resonation frequency.
JP-A-H06-224678 or JP-A-2005-65042 discloses a method of changing a resonation frequency of a surface acoustic wave element so as to make the element to have a target value. In JP-A-H06-224678, an electrode pattern is formed, and a dry etching is performed to a substrate using an electrode as a mask. While the dry etching is performed, the resonation frequency is measured. Thus, the resonation frequency can be changed by increasing a virtual thickness of the electrode. In JP-A-2005-65042, an electrode pattern is formed, and a side face of a comb-teeth electrode is anodized. Thus, the resonation frequency can be changed by increasing a weight and a width of the comb-teeth electrode.
However, because the comb-teeth electrode and the reflector are formed by using the photo etching, minute adjustment of the electrode interval cannot be performed with an accuracy higher than that of the photolithography. Therefore, if the target resonation frequency is set by changing the electrode intervals uniformly, a changing unit of the resonation frequency is limited.
For example, in a case that one-to-one exposure is performed, the minimum value of the electrode interval controllable by the photolithography is equal to a resolution of a reticle. In a case that LiNbO3 substrate is used, when the reticle resolution is about 0.1 μm, the resonation frequency can be changed only by a unit of 2 MHz. However, the resonation frequency is required to be changed in more minute units.
The resonation frequency is set after forming the electrode pattern; in JP-A-H06-224678 or JP-A-2005-65042. That is, the required resonation frequency cannot be set at a design step of the electrode pattern.
The controlling method of the resonation frequency disclosed in JP-A-H06-224678 or JP-A-2005-65042 has both of advantages and disadvantages. The controlling method is required to have a variety of choices. If the controlling method can be selected among the choices based on the purpose, the resonation frequency can be easily and accurately changed.